Many methods are known for manufacturing a conjugated diene polymer having a high content of cis-1,4 structures, and particularly systems combining a transition metal compound such as titanium, cobalt, nickel, neodymium, etc. and an organoaluminum compound are often used. Pamphlet of International Publication No. 06/049016 (Patent Document 1) discloses a method of manufacturing a conjugated diene polymer having a high content of cis-1,4 structures by a catalyst utilizing an yttrium compound containing a bulky ligand. Normally, a linear conjugated diene polymer with few branches is obtained from such catalytic systems.
However, such a conjugated diene polymer with few branches has a high solution viscosity and a high melt viscosity, and might be difficult to work for some application purposes. To improve from this problem, it has been considered to introduce branch structures into the polymer.
For example, JPH7-2959A (Patent Document 2) discloses a method of manufacturing a branched conjugated diene polymer by reacting a living polymer which is obtained by polymerizing a conjugated diene by using organolithium as an initiator with alkoxysilane as a coupling agent.
JPH2-45508A (Patent Document 3) discloses a method of manufacturing polybutadiene by using a cobalt-type catalyst, wherein the method intends to reduce the solution viscosity of the polybutadiene while maintaining its content of cis-1,4 structures high by adjusting the amounts of use of water and a chain transfer agent.
JP2004-204229A (Patent Document 4) discloses a method of manufacturing a branched conjugated diene polymer having a high content of cis-1,4 structures and a low solution viscosity by polymerizing a conjugated diene at 70 to 140° C. by using a catalyst containing a lanthanide compound.
Polybutadiene, which is one of conjugated diene polymers, is known as a rubber having excellent mechanical properties and thermal properties and used for various purposes including tires. With rising concern for environmental issues in the recent years, a demand for promoting fuel efficiency of also tires has become increasingly stronger. It has been found that in order to promote the fuel efficiency of tires, it is effective to use a rubber composition having a low loss tangent (tan δ) (a low loss property). As specific methods for this, many methods have been proposed that use silica as a reinforcing agent.
However, using silica as a reinforcing agent causes a problem of reducing abrasion resistance, mechanical properties, etc. as compared with when using carbon black, because dispersibility of silica in a rubber composition is poorer than that of carbon black. Regarding this, Patent Document 5 discloses a method for improving dispersibility of a reinforcing agent by using a modified polymer.